Microfluidic devices have recently become popular for performing analytic testing, fast screening reactions, but also many other biological and biomedical applications. Microfluidic technology can be used to deliver a variety of in vitro diagnostic applications at the point of care, including blood cell counting and characterization, calibration-free assays directly in whole blood; applications in drug discovery, synthetic chemistry, and genetic research. A further application resides in processing life-based organic particles including particles selected from the list comprising cells, cellular spheroids, tissues, eukaryotes, micro-organisms, organs or embryos, as for instance disclosed in WO-A-2012/120102, for e.g. for screening of new medical treatments.
There is growing scientific and industrial interest in capillary pressure barriers for controlling or influencing the behaviour of fluids, especially liquids or liquid-containing substances. Such capillary pressure barriers are of particular utility in the field of microfluidics, in which they are highly useful in controlling the flow of bodies of liquids in volumes the sizes and shapes of which are designed for specific purposes such as assaying, “aliquoting” i.e. the dispensing to or from a volume of a predetermined quantity of a liquid, mixing, separating, confining metering, patterning and containing. Effective passively exerted fluid flow control has become greatly sought-after to controlling liquids in large microfluidic circuits and liquids in microfluidic chambers. It would even be more desirable to be able to trigger such valves, such as by a second fluid. Published literature mentions valves in which two menisci trigger each others' advancement with the help of pinning barriers. US2005 0118070A1 describes a flow triggering device. In this disclosure a liquid meniscus is pinned on a pinning barrier. A second meniscus arrives after a delay and relieves the pinned state of the first liquid meniscus. This arrangement only works if one is sure of the order of arrival of the two liquid menisci. If this occurs in reverse order the second liquid does not become pinned and air will be entrapped in the device.
J. Melin et al., in “A liquid-triggered liquid microvalve for on-chip flow control”, Sensors and Actuators B 100 (2004) 463-468, disclose a device in which either meniscus is pinned, whichever arrives first. Pinning is realized by two sharp edges that are patterned in the z-direction. Since the two pinning barriers have one sharp edge in common, the second meniscus, upon arrival, touches the first and thereby relieves the pinned state. A similar type of valve was disclosed by M. Zimmermann et al., “Valves for autonomous capillary systems”, Microfluid Nanofluid (2008) 5:395-402
The fact that the devices require a common pinning barrier for allowing the double pinning function, while maintaining the overflow capacity upon presence of both meniscii, means that the common pinning barrier needs to be patterned in the z-direction, that is orthogonal to the plane on which the microfluidic network is patterned. This has the disadvantage that the pinning barrier is in practice always part of the microfluidic channel structure, and therefore compromises need to be made to the channel geometry. In fact, both publications require complex geometries that are implemented with deep reactive ion etching of silicon: an expensive, time-consuming procedure.